The invention relates to an unbalanced shaft for the compensation of inertial forces and/or mass moments of inertia of an internal combustion engine. The unbalanced shaft comprises an unbalanced portion, the mass center of gravity of which runs eccentrically with respect to the axis of rotation of the unbalanced shaft for generating the shaft imbalance, a bearing portion adjacent to the unbalanced portion and a bearing ring which is attached to this and the outer surface area of which serves as a raceway for the rolling bodies of a rolling bearing rotationally supporting the unbalanced shaft, and the inner surface area of which, in a bearing circumferential region extending in the imbalance direction, is set back radially with respect to the outer enveloping circle of the unbalanced portion. The invention relates, furthermore, to a method for producing such a shaft.
The increasing need to reduce the generation of friction and consequently change over from unbalanced shafts mounted hydro-dynamically on plain bearings to those mounted on rolling bearings for the purpose of the compensation of inertial forces in an internal combustion engine necessitates structural changes to the shafts which take into account the specific rolling load upon the bearing points. The requisite rolling strength can usually only be achieved, in the case of shafts produced in one part, by forging, this production method entailing higher costs, as compared with the cast steel shafts mounted on plain bearings.
A multipart, as it were built-up, unbalanced shaft of the type initially mentioned may be gathered from DE 10 2008 018 545 A1. This proposes a shaft mounted on rolling bearings and made from cast material, with a bearing ring made from rolling bearing steel, and in this case the bearing ring which is partially or completely set back radially with respect to the adjacent unbalanced portion is introduced into the casting die and the shaft material is subsequently cast around it. However, such a method for attaching the bearing ring on the associated bearing portion of the unbalanced shaft may entail some disadvantages. Thus, the introduction of the bearing ring into the casting mold requires a correspondingly adapted casting process. Moreover, the cast-in bearing ring has to be re-machined mechanically for the purpose of its necessary surface quality and has to be retreated thermally for the purpose of the required surface hardness.